1. Field of the Invention
This invention generally relates to an optical information processing device and particularly to an optical pickup which receives a light beam reflected from a recording medium to control its position with respect to the recording medium. More specifically, the present invention relates to an optical pickup for carrying out tracking and focusing at high accuracy.
2. Description of the Prior Art
An optical pickup for optically reading information stored in an optical disk is well known. Such an optical pickup is normally structured to carry out tracking and focusing controls so as to have a light beam properly follow and stay focused on a track along which information is recorded. For this purpose, the optical pickup receives a light beam reflected from the optical disk to produce a position control signal, which is used to control the position of the optical pickup with respect to the optical disk. One of the methods for detecting the position of the optical pickup with respect to the disk is the knife-edge method.
FIGS. 1a and 1b show the overall structure of the typical prior art optical pickup system for detecting tracking and focusing conditions on the basis of the knife-edge method. As shown, a light beam emitted from a light source 1, such as a semiconductor laser, is collimated (parallel) by a coupling lens 2 and the thus collimated beam is reflected by a polarizing beam splitter 3 to pass through a 1/4 wave length plate 4. The light beam is then focused by an objective lens 5 onto the surface of a disk 6 thereby forming a spot having the diameter of approximately 1.6 microns.
A reflected light beam from the surface of the disk 6 passes through the objective lens 5 to be also converted into a collimated light beam which is then converted into a linearly polarized light beam whose plane of polarization is perpendicular to that of the incident light beam by the 1/4 wave length plate 4. Thereafter, the light beam passes through the beam splitter 3 and is made convergent by a lens 7. As shown in FIG. 1b, a half of this convergent light beam 10 is incident on a tracking error detecting device 8 which is comprised of a pair of light receiving elements C and D arranged on both sides with respect to the direction of track T, and the remaining half of the beam 10 is incident on a focus error detecting device 9 comprised of a pair of light receiving elements A and B arranged on both sides of a knife edge defined by the tracking error detecting device 8.
The principle of focusing error detecting operation of the optical pickup shown in FIG. 1a will be described with reference to FIGS. 2a through 2c. The top end of the tracking error detecting device 8 serves as a knife edge, and, under the in-focus condition as shown in FIG. 2a, outputs from the respective light receiving elements A and B are equal. However, when the disk 6 moves away from the objective lens 5 as shown in FIG. 2b, the output from the element A becomes smaller than the output from the element B; on the other hand, when the disk 6 moves closer to the objective lens 5 as shown in FIG. 2c, the output from the element A becomes larger than the output from the element B. In this manner, the focus error condition may be detected by comparing the outputs from both of the elements A and B.
Regarding the tracking error detecting operation, when the spot is formed in registry with a track as shown in FIG. 3a, outputs from the respective elements C and D are equal. However, as shown in FIG. 3b when the spot is shifted from a track, outputs from the respective elements C and D become unequal. That is, one of the outputs become larger than the other depending upon the direction of shift with respect to the track.
In such a prior art optical pickup, accuracies required for focusing error detection and for tracking error detection are 1 and 0.1 micron, respectively. In the prior art knife edge method, the tracking error detecting device receives approximately 50% of the light beam and the error in this signal is approximately 0.05 microns, which is approximately half of the required accuracy of 0.1 micron. This indicates that higher accuracies are required for other parts of the optical system, which tends to push up the cost.